1. Field of the Invention
The present invention relates to an apparatus and a method of measuring a vibration quantity, and a loop design method in which a vibration quantity generated in a focus and tracking loop is accurately measured to optimally design a focus and tracking loop of an optical disc drive.
2. Description of the Related Art
Where data is recorded on or read from an optical disc, an optical disc drive should accurately focus a recording surface of the optical disc with a focus servo, and have a pickup which accurately follows a track with a tracking servo. A focus and tracking loop should be accurately modeled to accurately control the focus and tracking servos. A focus and tracking controller that controls the focus and tracking loop should be designed so as to achieve a target performance of the optical disc drive.
To optimally model a focus and tracking loop, a vibration quantity occurring in the optical disc drive should be accurately measured. According to the measured vibration quantity and a tracking permissible error, a gain of the focus and tracking loop is determined. Therefore, where a vibration quantity occurring in each frequency can be accurately measured, a loop bandwidth at which the vibration quantity becomes the same as the tracking permissible error can be determined, and the focus and tracking loop can be optimally modeled by the accurately measured vibration quantity.
In prior art, a focus and tracking loop is modeled not by measuring the vibration quantity, but by determining the vibration quantity with a maximum vibration quantity and a maximum vibration acceleration predetermined in a specification. Here, the maximum vibration quantity occurs in a drive, and the maximum vibration acceleration occurs at the speed factor of one time. The specification should be satisfied by the optical disc rather than by the vibration quantity occurring in the drive. That is, after a disc is manufactured, where a maximum vibration quantity and a maximum vibration acceleration at the speed factor of one time are less than the values determined in the specification, the manufactured disc is regarded as a normal one.
Though the focus and tracking loops of all discs that satisfy the specification should be designed to satisfy the predetermined performance, most disc vibration quantities are less than a value determined in the specification. Therefore, where a focus and tracking loop is designed with these maximum vibration quantity and maximum vibration acceleration, a bandwidth or a gain of the focus and tracking loop becomes unnecessarily great, and an output of a block of the focus and tracking loop is saturated such that the focus and tracking loop is unstable.
To model a vibration quantity at the speed factor of n times, except for the speed factor of one time, according to a method used in modeling a vibration quantity at the speed factor of one time, the maximum vibration acceleration occurring at the speed factor of n times should be known. An acceleration is in proportion to the square of the speed factor. Therefore, where the maximum vibration acceleration at the speed factor of n times is obtained by multiplying the maximum vibration acceleration at the speed factor of one time by the square of n, the maximum vibration acceleration value at a high speed factor becomes too great such that there is no reliability in data. To solve this problem, where only the disc rotates, the maximum vibration acceleration in a focus direction for each speed factor is measured using a Laser Doppler Vibrometer (LDV), and may be used in modeling a focus vibration quantity. However, since it is difficult to measure the vibration in the tracking direction with the LDV, the maximum vibration acceleration for each speed factor cannot be measured.
Where a vibration of a predetermined frequency occurring in an optical disc drive is expressed by sine wave d(w) as in equation 1, the maximum acceleration a(w) can be obtained as a maximum value obtained by differentiating d(w) twice. Using the maximum acceleration, a vibration quantity d(w) for each frequency can be obtained by dividing the maximum acceleration by the square of the frequency.                                                                         d                ⁡                                  (                  w                  )                                            =                            ⁢                                                d                  w                                ⁢                sin                ⁢                                                                   ⁢                w                ⁢                                                                   ⁢                t                                                                                                        a                ⁡                                  (                  w                  )                                            =                            ⁢                              -                                  d                                      w                    ⁢                                                                                   ⁢                    sin                    ⁢                                                                                   ⁢                    w                    ⁢                                                                                   ⁢                    t                                                                                                                                          d                w                            =                            ⁢                                                a                  max                                                  w                  2                                                                                        (        1        )            
FIG. 1 shows a waveform diagram illustrating a minimum servo loop gain that can be calculated using a vibration quantity and a permissible error. Generally, the focus maximum vibration of a DVD disc determined in a specification is ±300 μm, and the maximum vibration acceleration at the speed factor of one time is 8 m/sec2. The maximum tracking vibration quantity is ±35 μm, and the maximum vibration acceleration at the speed factor of one time is 1.1 m/sec2. By dividing the maximum vibration acceleration by the square of a predetermined frequency, the vibration quantity occurring at the predetermined frequency can be obtained. Since the vibration quantity calculated at each frequency cannot be greater than the maximum vibration quantity, the frequency characteristic of the vibration quantity is as shown in FIG. 1.
The vibration quantity obtained by the equation 1 is used to obtain a minimum loop gain for a focus and tracking loop design. The minimum loop gain is defined as a value obtained by dividing a vibration quantity by a permissible error value. A minimum loop gain L0 of a low frequency area which is obtained by dividing the maximum vibration quantity by the permissible error, and a minimum loop gain L1 of a high frequency area which is obtained by dividing the vibration quantity, for each frequency, which is obtained using the maximum vibration acceleration, by the permissible error, are expressed as follows in equation 2, and have the frequency characteristics as shown in FIG. 1.                                                                         L                0                            =                            ⁢                              20                ×                                  log                  10                                ⁢                                                      d                    max                                                                              e                      max                                        ⁡                                          (                      w                      )                                                                                                                                                              L                1                            =                            ⁢                              20                ×                                  log                  10                                ⁢                                                      a                    max                                                                              w                      2                                        ⁢                                                                  e                        max                                            ⁡                                              (                        w                        )                                                                                                                                                                                                      e                  max                                ⁡                                  (                  w                  )                                            =                            ⁢                                                e                  max                                3                                                                        (        2        )            
A focus and tracking loop is designed according to the minimum loop gain obtained as in FIG. 1 so that a gain of the focus and tracking loop is greater than the minimum loop gain, and a target phase margin and a gain margin are obtained. Since the bandwidth of the focus and tracking loop indicates a frequency at which the vibration quantity becomes the same as the permissible error, the vibration quantity should be accurately measured to design a focus and tracking loop having an appropriate bandwidth according to a change in the speed factor.
However, in the prior art, a maximum vibration quantity and a maximum vibration acceleration are specifications for manufacturing discs, and may have no relation with a vibration quantity occurring in an actual drive. Therefore, it is highly probable that a focus and tracking loop is designed to have a gain greater than a needed gain. In addition, since it is not accurately known how much the maximum vibration acceleration increases in accordance with an increasing speed factor, a minimum loop gain cannot be calculated where the speed factor increases. Accordingly, a focus and tracking controller is designed repeatedly until a target performance is satisfied by a trial and error method through lengthy experiments.
A focus and tracking vibration quantity calculated by the prior art maximum vibration quantity and maximum vibration acceleration is always obtained as an identical value in identical types of discs. Therefore, when an actual vibration quantity varies, which depends on the type of a disc and drive, the focus and tracking vibration quantity cannot be reliable data. Since the vibration quantity of an identical disc and drive varies in every measuring time due to chucking or other reasons, it is necessary to measure the vibration quantity when the drive operates. According to the prior art, since it is highly probable that a loop gain is designed to be unnecessarily great, the focus and tracking loop is designed many times.